Heterogeneous, Platinum-Catalyzed Hydrogenation of (Diolefin)dialkylplatinum(II) Complexes: Kinetics

Article abstract of DOI:10.1021/ja00218a024

(Diolefin)dialkylplatinum(II) complexes <(Ol2)PtR2> are rapidly reduced by dihydrogen in the presence of platinum black catalyst: the organic groups are converted to alkanes, and the platinum(II) to platinum(0).This platinum(0) is incorporated into the surface of the platinum black catalyst.The reaction is a heterogeneous process catalyzed by platinum(0).Its rate is strongly influenced by mass transport and by the surface area of the catalyst.It is poisoned by dineopentylmercury, di-n-octyl sulfide, and tri-tert-butylphosphine.Because the catalyst surface is constantly renewed by deposition of platinum, the reaction is, however, less sensitive to poisoning than more familar platinum-catalyzed reactions such as hydrogenation of olefins.The form of the kinetic rate law observed for reduction of (1,5-cyclooctadiene)dimethylplatinum(II) (1) depends on the reaction conditions.Two limiting kinetic regimes are observed.In one, mass transport of dihydrogen to the catalyst surface is rate-limiting; in the second, a reaction occurring on the platinum surface is rate-limiting.The activation energy for reaction in the mass transport limited regime is ca. 3 kcal/mol and that in the reaction rate limited regime is ca. 15 kcal/mol.In neither regime is there a kinetic isotope effect: ν_H2/ν_D2 ca. 1.0.Examination of the relative rates of reaction of a series of (diolefin)dialkylplatinum(II) complexes indicates that the structure of the diolefin has a greater influence on the rate of reaction than does that of the alkyl group.Competitive experiments show that rates are influenced by adsorption of the (Ol2)PtR2 complex to the catalyst.These studies suggest that the platinum-catalyzed reaction of (Ol2)PtR2 with dihydrogen takes place by a mechanism analogous to that of the platinum-catalyzed heterogeneous hydrogenation of olefins: the (Ol2)PtR2 complex chemisorbs on the surface of the platinum catalyst; its organic groups are converted to platinum surface alkyls; coupling of these surface alkyls with surface hydrogen atoms generates the alkane products.The platinum atom originally present in the soluble complex is incorporated into the surface of the platinum catalyst and becomes the reactive site for a subsequent cycle of chemisorption and reaction.The reaction of (Ol2)PtR2 complexes with dihydrogen thus provides a new method for generating platinum-surface alkyls under conditions representative of those used in heterogeneous platinum-catalyzed processes.